Two-stroke engines

ABSTRACT

The specification discloses a stepped-piston two-cycle engine in which the cylinders are arranged in one or more sets of two cylinders. The charge is pumped from a pumping part of each cylinder of the or each set to a working part of the other cylinder of the set through separate first and second separate transfer passages opening into the working part through first and second inlet ports respectively which are arranged symmetrically relative to the exhaust port of the cylinder. The first transfer passages of each set cross and the second transfer passages of the set cross. Each cylinder is arranged between parts of the transfer passages which feed its working part. The arrangement gives a compact engine with very good scavenging and capable of high performance.

I Umted States Patent 1151 3,641,987

Hooper Feb.-l5, 1972 [54] TWO-STROKE ENGINES Primary ExaminerBenjamin W.Wyche [72] Inventor. Bernard Hooper, Maybank House, Hope AssistantExaminer Richard Rothman Street, Wordsley, Stourbndge, England Atmmey mmKelman 221 Filed: Mar. 2, 1970 57 ABS CT 211 App]. No.: 15,799 1 Thespecification discloses a stepped-piston two-cycle engine in which thecylinders are arranged in one or more sets of two [30] ForeignApplication I cylinders. The charge is pumped from a pumping part ofeach Mar. 7, 1969 Great Britain ..l2,077/69 cylinder "1 each 3 Partcylinder of the set through separate first and second separate transferpassages opening into the working part through first [1.8. CI. ..l23/71RF02, 33/14 and second inlet ports respectively which are arrangedsymmetrically relative to the exhaust port of the cylinder. The firstFleldotSearch ..l23/7l R 71V 59 BS 65 S transfer passages of each setcross and the second transfer was passages of the set cross. Eachcylinder is arranged between [561 Refer CIM parts of the transferpassages which feed its working part. The UNITED STA'IES PATENTSarrangement gives a compact engine with very good scaveng- 1 157 30s10/1915 11 123/59 BS ing and capable ofhigh pefomance' I ogan I3,301,237 1/1967 Wolf ..l23/7l l1 Claims,9Drawing Figures -s5- 3 so m 30e so I m 4D Qfi I I82 4 31331655 38 75 N 0,8" 1o c -c2 26 GI 11 &

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sum 5 or e BACKGROUND OF THE INVENTION 1. Field of the Invention Theinvention is concerned with stepped-piston two-stroke engines.

2. Description of the Prior Art This invention relates to multicylinder,two-stroke engines of the type comprising at least one set of twocylinders, each such cylinder having a working part of smaller diameterand a pumping part of larger diameter; a piston slidably reciprocable ineach cylinder and having a working part which slides in the working partof the cylinder and a pumping part which slides in. the pumping part ofthe cylinder; and transfer passage means between the pumping part ofeach cylinder of the or each set and the working part of the othercylinder of the set so that, in the or each set, each charge of air, orfuel and air mixture, required by the working part of each cylinder ofthe set is pumped, during operation of the engine, to such working partalong the transfer passage means from the pumping part of the othercylinder of the set.

Hereinafter, engines of the above type are referred to as being of thetype specified and the respective cylinders of the or each set arereferred to as the first and second cylinders of the set respectively.

It is an objectof the invention to provide an engine of the typespecified having conveniently arranged transfer passage means.

SUMMARY OF THE INVENTION According to the invention there is provided anengine of the type specified wherein the working part of each cylinderhas exhaust port means and first and second inlet port means arrangedsymmetrically about the longitudinal axis of the cylinder with respectto the exhaust port means, the pumping part of each cylinder deliversinto transfer passage means which, for the or each set comprises fourtransfer passages which are separate for at least the major portions oftheir lengths and consist of two first transfer passages extendingrespectively between the first inlet port means of the first and secondcylinders and the pumping parts of the second and first cylinders andtwo second transfer passages extending respectively between the secondinlet port means of the first and second cylinders and the pumping partsof the second and first cylinders, each cylinder being interposedbetween parts of the first and second transfer passages which feed theworking part of that cylinder, the first transfer passages of the setmutually crossing and the second transfer passages of the set mutuallycrossing and wherein portions of substantial length of the first andsecond transfer passages immediately adjacent to the first and secondinlet port means of each working part are disposed so that the flowpaths of the charge entering said working part through said first andsecond port means converge and meet adjacent a deflecting surfaceopposite to the exhaust port means and which directs the charge towardsthe cylinder head of said working part.

In addition to the first and second inlet ports each working part mayhave an auxiliary inlet port connected to one or both of the transferpassage or passages feeding the working part by an auxiliary transferpassage of lesser cross-sectional area than the or each transfer passagefrom which it is fed and opening into the working part through saidauxiliary inlet port which is opposite to the exhaust port means. Thefirst and second transfer passages of each working part may be mutuallyconnected by a passage of less cross-sectional area than the transferpassages to balance any small pressure differences which may arisebetween the passages.

The transfer passages are substantially similar so that the amounts ofcharge introduced through the first and second inlet port means of aworking part in a cycle are substantially equal and enter the workingpart at substantially equal speeds.

Preferably also the first transfer passages lie wholly on one side of acentral plane (as hereinafter defined) and the second transfer passageslie on the opposite side of said plane.

By central plane" is meant, for the or each set, a plane containing thelongitudinal axes of the cylinders of the set and which either (1)contains the axis of the engine crankshaft where the cylinders arearranged side by side or in opposition or (2) is perpendicular to theaxis of the engine crankshaft when the cylinders of the set are arrangedinopposition or in V-formation.

In stating that the transfer passages are separate" there is meant thatthe passages do not share any common boundary surfaces substantiallythroughout their lengths. The expression port means is used to include asingle port or a plurality of ports which communicate with the sametransfer passage or exhaust passage.

The advantage of this arrangement is that symmetrical and efficientscavenging of the working parts, which is essential for highperformance, can be obtained in a compact engine. The arrangement alsoallows symmetrical exhaust outlets to be provided which is necessary onmotorcycle engines. The provision of separate transfer passages allowsthese to be of adequate cross-sectional area and said portions thereofto be shaped to give the incoming charge the desired direction of flow.

The engine may include a unitary cylinder block and crankcase and allthe transfer passages maybe formed in the unitary cylinder block andcrankcase during casting thereof in a manner similar to the formation ofwater passages in cylinder blocks of water-cooled engines or thetransfer passages may be formed partly in the crankcase and partly inseparate cylinder blocks. If either of these constructions is used, thecrankcase or unitary cylinder block and crankcase for the or each setmay be made in two halves having mating faces arranged so that thepassage parts mate when the two halves are secured together and formsaid four separate transfer passages.

For making two-stroke motorcycle engines it is preferred, wherepossible, to make the parts by diecasting and for such engines it'ispreferred to use a construction wherein each set of cylinders comprisesa crankcase having recesses formed therein, a cylinder block or cylinderblocks formed with recesses therein and a plate interposed between thecrankcase and the cylinder block or blocks, the plate and the recessescooperating to form at least parts of said four separate transferpassages. It is preferred to provide the swept volumes of the pumpingparts of the cylinders in the crankcase and the working parts each in aseparate cylinder block so as to facilitate alignment of the working andpumping parts of each cylinder, such cylinder block also containing theclearance volume of the associated pumping part. When using a plate,where the first and second passages cross they will be separated fromeach other by the plate.

In a still further arrangement, parts of the passages are formed in thecrankcase and/or cylinder block or blocks; the passages opening to theexternal walls of the crankcase and/or cylinder block or blocks to whichare secured crossover means which interconnect said parts.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be describedin-detail by way of example with reference to the accompanying drawingsin which;

FIG. 1 is an exploded perspective view of parts of the crankcase andcylinder blocks of a two-cylinder engine constituting a first embodimentof the invention and showing a perspective view from above of a platebetween the crankcase and the cylinder blocks;

FIG. 2 is a perspective view from below of the plate of FIG.

FIG. 3 is a vertical section through one of the cylinders of the engineparts of which are shown in FIGS. 1 and 2;

FIG. 4 is a section on the line lV-IV of FIG. 3 through the workingparts of both cylinders with the pistons omitted;

FIG. 5 is an exploded perspective view of the crankcase and cylinderblocks of a second embodiment of the invention;

FIG. 6 is an exploded perspective view, with the cylinder heads removed,of a third embodiment of the invention; and

FIGS. 7, 8 and 9 are diagrams illustrating modifications of the transferpassage connections between the pumping part of one cylinder of a setand the working part of the other cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 to 4,the engine comprises a crankcase indicated generally at 10 and first andsecond cylinder blocks 11 and 12, each cylinder block having a cylinderhead, that for the block 12 being indicated at 13. Each cylinder of thetwo-cylinder engine is formed of two parts, a working part of smallerdiameter and a pumping part of larger diameter. Referring to the firstcylinder this has a working part 15 and a pumping part 16. The secondcylinder has a working part 17 and a pumping part 18. The constructionof the two cylinders is identical and will be described in relation tothe second cylinder and to FIGS. 1 and 3. Thus the working part 17 isformed within a liner 20 received within the cylinder block 12 and thepumping partlS of the cylinder is formed mainly within a liner 21received within the crankcase l0 and projecting proud of the uppersurface 22 of the crankcase'to have parts 23 with upper surfacessubstantially level with the upper surface of a plate 24, which isinterposed between the crankcase l0 and the cylinder blocks 11 and 12,and to have parts 25 which terminate within the thickness of the plate24 and define the lower boundaries of outlet ports 26 and 27 whose upperboundaries are formed by the radially inner ends 28a'and 29a of recesses28 and 29 formed in-the lower surface of the cylinder block 12. Theserecesses commucate at their radically inner ends with the annularclearance volume 180 ofthe pumping part 18 of the cylinder. The lowerend of the cylinder block 12 is formed with spigot parts 30 which fitwithin the parts 23 and thus locate the cylinder block 12 relative tothe liner 21 and thus locate the liner 20 relative to the liner 21 thusmutually locating the working and pumping parts of the cylinder.

In a similar manner the working part 15 is formed within a liner 31 andthe pumping part 16 is formed mainly within a liner 32, the cylinderblock 11 having spigot parts 33 which locate within parts 34 at theupper end of the liner 32, such parts having upper surfaces level withthe upper surface of the plate 24, such upper end also having parts 35which terminate within the thickness of the plate 24 and define thelower boundaries of outlet ports 36 and 37 whose upper boundaries aredefined by the radiallyinner ends 38a and 39a of recess 38 and 39 formedin the cylinder block 11 and communicating with the clearance volume 16aof the pumping part 16.

Rotatably mounted in the crankcase 10 is a crankshaft indicatedgenerally at 40 having three journal bearings (not shown), the outerbearings being at the ends of the crankcase and the central bearingbeing in a central web 41 of the crankcase. The crankshaft has twothrows each receiving the big end of a connecting rod the small end ofwhich is connected to a piston reciprocable in one of the cylinders.Each piston has a working part and a pumping part. Thus referring to thesecond cylinder, the piston indicated generally at 42 has a working part43 which is slidable in the working part 17 of the cylinder and thepiston has a pumping part 44 which is slidable in the pumping part 18 ofthe cylinder. The working part 43 carries a gudgeon pin 45 and aconnecting rod 46 extends between the gudgeon pin and the crankshaft. Ina similar manner, the first cylinder has a piston (not shown) which hasa working part slidable in the working part 15 and a pumping partslidable in the pumping part 16 and connected by a connecting rod to thecrankshaft.

Transfer passage means, now to be described, are arranged so that duringoperation of the engine, charges are pumped from the pumping part 16 ofthe first cylinder to the working part 17 of the second cylinder andcharges are pumped from the pumping part 18 of the second cylinder tothe working part 15 of the first cylinder. Loop scavenging of theworking parts of the cylinders is also obtained as will be described.

Referring now to FIG. 4, the working part 15 has a first inlet port 47,a second inlet port 48 and an exhaust port 49. Similarly, the secondworking part 17 has a first inlet port 50, a second inlet port 51 and anexhaust port 52. It will be seen that in each working part the inletports are arranged symmetrically about the longitudinal axis of thecylinder with respect to the exhaust port. Loop scavenging is obtainedduring operation of the engine the fresh charge entering through theinlet ports and converging and meeting adjacentthe inner surface of theworking part opposite the exhaust port and being deflected by saidsurface towards the top of the cylinder to displace the burnt chargewhich flows downwardly and through the exhaust port. It will be notedthat the first inlet ports 47, and 50 lie to one side of a central planeindicated by the line 53. The central plane contains the longitudinalaxes of the cylinders and the rotary axis of the crankshaft. The exhaustports 49 and 52 communicate with exhaust passages 54 and 55 respectivelywhich may be connected to exhaust pipes. It is to be noted that theexhaust passages 54 and 55 are symmetrically arranged relative to a lineperpendicular to the line 53 and midway between the axes of the two'cylinders.

Fresh charges are introduced into the engine through inlet passages 56and 57 leading respectively to the recesses 29 and 39 and thus to thepumping parts 16 and 18 through the ports 37 and 27. As shown in FIG. 3,for the inlet passage 57, each inlet passage may be fitted with an inletfitting 58 containing a reed valve 59 and connected to the flange 60 ofa carburettor, not shown.

Transfer passage means extend between the pumping part 16 of the firstcylinder and the working part 17 of the second cylinder and between thepumping part 18 of the second cylinder and the working part 15 of thefirst cylinder. These transfer passage means comprise two separatetransfer passages between each pumping part and its associated work ingpart. The transfer passages include the recesses 28, 29, 38 and 39already described and are formed by passages in said blocks, aperturesand recesses in the plate 24 and recesses in the crankcase 10. 4

The flow'path of a charge along each of the four transfer passagesprovided is shown in FIG. 1 by a series of arrows, the arrows beingdistinctively shown.

Dealing first with the flow from the pumping part 18 of the secondcylinder to the working part 15 of the first cylinder the flow of acharge along the first transfer passage is shown by the ribbon arrows 61and the flow of a charge along a second transfer passage is shown by thedashed arrows 62. The first transfer passage along which the flow isindicated by the arrows 61 is constituted by the recess 29, an aperture63 in the plate 24, a recess 64 in the upper surface 22 of thecrankcase, a further aperture 65 in the plate 24 and a passage 66 in thecylinder block 11 leading to the first inlet port 47 of the working part15. The aperture 63 is aligned with one end of the recess 64 and theaperture 65 is aligned with the other end thereof, the plate 24 closingthe recess between said apertures 63 and 65. The undersurface of theplate 24 is recessed at 67 to give a greater cross-sectional area forthe flow of the charge along the recess 64.

The second transfer passage from the pumping part 18 to the working part15 directs the charge along the path indicated by the arrows 62 and isconstituted by the recess 28, an aperture 68 in the plate 24, a recess69 in the upper surface 22 of the crankcase, a further aperture 70 inthe plate 24 and a passage 71 in the cylinder block 11' leading to thesecond inlet port 48. The aperture 68 is aligned with one end of therecess 69 and the aperture 70 is aligned with'the other end of therecess, the plate closing off the recess 69 between the ends thereof butbeing recessed on its underside at 72 to increase the cross-sectionalarea of the recess 69 available for the fiow of charge therealong. Theupper surface of the plate 24 is relieved at 73 adjacent the aperture 63so as to be aligned with the upper surface of one of the parts 25 of theliner 2] and the upper surface of the plate 24 is also relieved at 74adjacent the aperture 68 to be aligned with the upper surface of theother nnnn M- part 25. The relief at 74 has a smoothly curved surfaceleading to the aperture 68 to give good flow characteristics.

Dealing now with the flow from the pumping part 16 of the first cylinderto the working part 17 of the second cylinder there are again twoseparate transfer passages, the flow along the first passage beingindicated by the full line arrows 75 and the flow along the secondpassage being indicated by the chain line arrows 76.

Dealing first with the transfer passage represented by the arrow 75,this passage is constituted by the recess 39, and is constituted by anaperture 77 in the plate .24, a recess 78 in the upper surface 22 of thecrankcase, a further aperture 79 in the plate 24 and a passage 80leading to the first inlet port 50 in the working part 17. The aperture77 communicates with one end of the recess 78 and the aperture 79 withthe other end of the recess, the recess being closed off by the platebetween said ends and the plate being recessed on its underside at 81 toincrease the cross-sectional area of the recess 78 available for flow.The upper surface of the plate 24 is relieved at 82 to bring it levelwith the upper surface of one of the parts 35 of the liner l6 and theupper surface of the relief is smoothly curved in a manner described inrelation to the relief 74.

The second transfer passage represented by the chain line arrows 76 isconstituted by the recess 38, an aperture 83 in the plate 24, a recess84 in the upper surface 22 of the crankcase 10, a further aperture 85 inthe plate 24 and a passage 86 leading to the second inlet port 51 of theworking part 17 One end of the recess 84 is aligned with the aperture 83and the other 'end of the recess is aligned with the aperture 85, therecess 84 being closed off by said plate between said ends but theunderside of the plate being relieved at 87 for the purpose hereinbeforedescribed.

The central plane referred to above is indicated by the line 53 in FIG.3 in addition to the line 53 in FIG. 4. It will be seen that the firsttransfer passages represented by 'the arrows 61 and 75 and which feedthe first inlet ports 47 and 50 lie wholly to one side of this plane andthat the second transfer passages represented by the arrows 62 and 76and which feed the second inlet ports 48 and 51 lie wholly to theopposite side of the plane represented by the line 53. By thisarrangement, it is possible to locate the inlet ports in the workingparts symmetrically as shown in FIG. 4 while still obtaining a compactengine and having the exhaust passages 54 and 55 also symmetrical whichis important for a motor cycle engine. It will also be seen that thefirst transfer passages whose flow is represented by the arrows 61 and75 cross over and are separated by the plate 24. Thus the crossoveroccurs as the charge from the pumping part 16 flows over the relievedportion 82 and into the aperture 77 thus crossing over the chargeflowing in the recess 64. Similarly, the second transfer passages crossover as the charge represented by the arrow 62 flows over the relievedportion 74 and thus over the charge flowing in the recess 84. It

will also be seen that the cylinder 15, 16 is interposed between.

parts of the transfer passages represented by the arrows 61 and 62 andthat the cylinder 17, 18 is interposed between parts of the transferpassages represented by the arrows 75 and 76.

It is believed that operation of the engine will be substantiallycomprehended from the foregoing description but one cycle will bedescribed. It will be appreciated that the piston in the first cylinderis 180 out of phase with the piston in the second cylinder, the onebeing at bottom dead center while the other is at top dead center.

Assuming that the first piston is at bottom dead center and the secondpiston is at top dead center, the pumping part of the first piston willhave induced a charge into the pumping part 16 of the first cylinderthrough the inlet passage 56 and through a reed valve similar to thatshown at 59. The charge in the second cylinder will have been ignited.As the piston in the first cylinder moves towards top dead center thecharge in the pumping part 16 thereof will flow as indicated by thearrows 75 and 76 towards the working part 17 of the second cylinder. Thecharge which has already been introduced into the workand the firstinlet port 101. This transfer passage is constituted ing part 15 of thefirst cylinder will be compressed. As the piston in the second cylinderdescends, when the working part of the piston uncovers the ports 50,51and 52, the charge which is being pumped from'the pumping part 16 willenter the working part 17, will flow upwardly towards the top of thecylinder and will displace the burnt charge which will flow out of theexhaust port 49. As the pumping part of the piston in the secondcylinder descends it will induce a charge into the pumping part 18through the'reed valve 59 and the recess 28. The charge in the firstcylinder will now be exploded and as the piston in the second cylindermoves towards top dead center the charge in the pumping part 18 will bedischarged through the outlet ports 26 and 27 and will flow as indicatedby the arrows 61 and 62 towards the working part 15 of the firstcylinder. When the working part of the piston in the first cylinderuncovers the ports 47, 48 and 49 the charge will enter the cylinderthrough the ports 47 and 48 and will displace the burnt charge throughthe exhaust port 49.

The portions of the transfer passages formed by the passages 66, 71, and'86 are so shaped as to lead the charge into the working parts in thedesired directions to obtain loop scavenging as described above.Moreover care is taken that the passages 66 and 71 shall be similar insize and shape so that the passages 80 and 86 shall be similar in sizeand shape so that a balanced flow of charge enters the working part.

Various modifications of the arrangement described in relation to FIGS.1 to 4 are possible and one of these modifications is shown in FIG. 5which illustrates diagrammatically an arrangement in which there is noplate such as 24 and the transfer passages are formed partly in twohalves of a crankcase which mate along a plane perpendicular to therotary axis of the crankshaft, and partly in the cylinder blocks. Thecylinder block for the crankcase half 90 is indicated at 92 and thecylinder block for the crankcase half 91 is indicated at 9 3. A firstcylinder is formed in the parts 90 and 92 and has a pumping part 94 anda working part 95. The working part has an exhaust port 96 and first andsecond inlet ports of which only the first such port is shown at 97. Theinlet ports are arranged symmetrically relative to the exhaust port in amanner similar to that described in relation to FIG. 4. The cylinderformed in the parts 91 and 93 has a pumping part 98 and a working part99. The working part has an exhaust port 100 and first and second inletports arranged symmetrically relative to the exhaust port 100, only thefirst inlet port being shown and being indicated at 101.

Four transfer passages are provided as before thus there is a firsttransfer passage the flow along which is indicated by the full linearrows !02 and extends between the pumping part 94 by a recess 103 inthe lower surface of the cylinder block 92 and a cooperating recess 104in the upper surface of the crankcase half 90 (which recessescommunicate with the clearance volume 94a of the pumping part 94) apassage 105 leading to a plane face 106 of the half 90, a passage 107leading from a plane face 108 of the half 91 and a passage 109 in thecylinder block 93 leading to the first inlet port 101.

There is a first transfer passage leading from the pumping part 98 tothe working part 95 and the flow along this passage is indicated by theribbon arrows 110. The transfer passage is constituted by a recess 11 1in the lower surface of the cylinder block 93 and a cooperating recess112 in the upper surface of the crankcase half 91 and is constituted bya passage 113 extending to the face 108, a passage 114 in the crankcasehalf 90 extending from the face 106 and a passage 115 in the cylinderblock 92 extending to the first inlet port 97 There is a second transferpassage indicated by the chaindotted arrows 116 extending from thepumping part 94 to the working part 99. This transfer passage isconstituted by the recesses 103 and 104 and a passage 117 in thecrankcase half 90, a passage 118 in the crankcase half 91 and a passage119 in the cylinder block 93 which leads to the second inlet port,

not shown, in the working part 99. There is a second transfer passagefrom the pumping part 98 to the working part 95 and innvm this isconstituted by the recesses 111 and 112, a passage 120 in the crankcasepart 91, a passage 121 in the crankcase part 90 and a passage 122 in thecylinder block 92 leading to the second inlet port, not shown, of theworking part 95. The flow along this transfer passaged is represented bythe dotted line arrows 123.

It will be seen that the first transfer passages, the flows along whichare represented by the arrows 102 and 110 lie to one side of a centralplane containing the longitudinal axes of the cylinders and also therotary axis of the crankshaft although the latter is not shown. Thesecond transfer passages, the flows along which are represented by thearrows 116 and 123 lie to the other side of said central plane.

The recesses 103 and 111 in the cylinder blocks 92 and 93 respectivelycommunicate with inlet passages 124 and 125 respectively which in thecomplete engine will have reed-valve containing fittings securedthereto.

The transfer passages are separate throughout the major parts of theirlengths, being common only at the recesses 103, 104, and 111, 112. Itwill be seen that the first transfer passages represented by the arrows102 and 110 cross and the second transfer passages represented by thearrows 116 and 123 cross. The cylinder 94, 95 is interposed betweenparts of the passages 1 10 and 123 and the cylinder 98, 99 is interposedbetween parts of the passages 102 and 116.

The operation of the embodiment of FIG. is similar to the operation ofthe embodiment of FIGS. 1 to 4 and will therefore not be described.

FIG. 6 shows a further modification. In this modification, the enginecomprises a cylinder block 130 in which are formed a first cylinder 131and a second cylinder 132. The first cylinder 131 has a working part133and a pumping part 134 and the second cylinder has a working part 135and a pumping part not visible in the drawing. A piston 136 isreciprocable in the cylinder 131 and has a working part 137 and apumping part 138. The piston is connected to a crankshaft 139 by meansof a connecting rod 140. The working part 133 has a first inlet port141', a second inlet port 142 and a exhaust port 143, the ports 141, 142and 143 being arranged for loop scavenging as described above; thepumping part 134 has port means 144. The first inlet port 141 is on oneside of a plane indicated by the line 145 through the longitudinal axesof the cylinders and the axis of the crankshaft 139. The working part135 similarly has first and second inlet ports on the same sidesrespectively of the plane 145 as the inlet ports 141 and 142 of theworking part 131. The working part 135 also has an exhaust port arrangedin a manner similar to the exhaust port 143.

The inlet ports and the outlet ports communicate with the external wallsof the cylinder block and the crossing over of the four transferpassages which are provided and which are arranged as described below iseffected by first and second transfer members 146 and 147 operating inconjunction with passages and recesses formed in the external walls ofthe cylinder block and cover members 148 and 149.

There is a first transfer passage, the flow along which is indicated bythe full line arrow 150 between the pumping part 134 and the workingpart 135. This transfer passage extends from the clearance volume 151 ofthe pumping part 134 and comprises a Z-shaped recess 152 in a transfermember 147 and a passage, not shown, in the cylinder block leading fromthe recess 152 to the first inlet port of the working part 135.

There is a first transfer passage from the pumping part of the cylinder132 to the first inlet port 141 in the working part 133, the flow alongthis transfer passage is indicated by the ribbon arrow 153. The transferpassage starts from the recess in communication with the pumping part ofthe cylinder 132, not shown, and comprises an aperture 154 in thetransfer member 147, a Z-shaped recess 155 in the cover member 149,another aperture 156 in the transfer member 147 and a passage 157 in thecylinder block leading to the first inlet port 141.

There is a second transfer passage leading from the pumping part 134 tothe working part 135 and the flow path of a charge along this passage isindicated by the chain-dotted arrow 158. The passage is formed by arecess 158a in the cylinder block which is closed by a wall 159 in thetransfer member 146, the recess 158a leading to the second inlet port ofthe working part 135. The recess 158a communicates with an aperture 160in the wall 159 which in turn communicates with an aperture 161 in thecover member 148. The aperture 161 is adapted to be connected to acarburettor to allow a charge to be induced into the pumping part 134through the apertures 161, 160 and through the port means 144.

There is a second transfer passage from the pumping part of the cylinder132 to the second inlet port 142 of the working part 133. The flow pathof a charge in this transfer path is indicated by the dotted arrow 162.The transfer path is formed by a passage 163 which communicates with thepumping part of the cylinder 132, an aperture 164 in the wall 159, arecess 165 in the transfer member 146 and which is closed by the coverplate 148, a further aperture 166 in the transfer member 146 and apassage 167 leading to. the second inlet port 142.

In operation the pumping part 134 pumps charges along the first andsecond transfer passages represented by the arrows 150 and 158 to theworking part 135 and the pumping part of the cylinder 132 pumps chargesalong first and second transfer passages represented by the arrows 153and 162 to the working part 133. The first transfer passages 150, 153are on one side of the plane indicated by the line 145 and the secondtransfer passages 158, 162 are on the other side of said plane. Thetransfer passages represented'by the arrows 150 and 153 cross and thetransfer passages represented by the arrows 158 and 162 cross. Thecylinder 131 is interposed between parts of the passages 150 and 158 andthe cylinder 132 is interposed between parts of the passages 153 and162.

FIGS. 7, 8 and 9 indicate diagrammatically modifications of thearrangement described in detail with'reference to FIGS. 1 and 6 and eachdiagram shows the transfer path connections between the working part ofone cylinder and the pumping part of the other. It is to be understoodthat the arrangement will be symmetrical and that the pumping part ofthe other I cylinder will communicate with the working part of the onecylinder through a similar arrangement of transfer passage means.

Referring to FIG. 7 the pumping part of the first cylinder is indicatedat 170 and the working part of the other cylinder at 171. First andsecond transfer passages 172 and 173 extend from the pumping part 170 tothe working part 171 and open into the latter through first and secondinlet ports 174 and 175 respectively. The inlet ports are arranged forloop scavenging with reference to an exhaust port 176. Thus far thearrangement is as described in relation to FIGS. 1 to 4. Themodification is that a balance passage 177 of lesser cross-sectionalarea than the transfer passages 172 and 173 is connected between thepassages. The purpose of the balance passage is to even out any smalldifferences in pressure there may be between the charges flowing alongthe transfer passages 172 and 173. The central plane is indicated by theline 178. I

Referring now to FIG. 8, the pumping part of one cylinder is indicatedat 179 and the working part of the other at 180. First and secondtransfer passages are indicated at 181 and 182 and open into first andsecond inlet ports 183 and 184 in the working part 180. The firsttransfer passage 181 and the first inlet port 183 are to one side of thecentral plane indicated by the line 185 and the transfer passage 182 andthe second inlet 184 are arranged on the other side of the plane 185.The working part has an auxiliary inlet port 186 which is intersected bythe plane and which is fed from the first transfer passage 181 by anauxiliary transfer passage 187. If desired the passage 187 may be fedalso from the passage 182 by a further auxiliary passage 187a showndotted. It will be seen that in this arrangement the main inlet portsare arranged symmetrically with respect to an exhaust port 188.

The auxiliary inlet port 186 gives cross scavenging towards the exhaustport 188.

FIG. 9 shows an arrangement which is similar to PK]. 8 except that theaxis of symmetry of the parts of the working part is inclined to thecentral plane. Thus referring to PK]. 9, the pumping part of onecylinder is indicated at 189 and the working part of the other at 190.First and second transfer passages are indicated at 191 and 192 and areon opposite sides of the central plane indicated by the line 193. Thefirst transfer passage 191 feeds the first inlet port 194 and the secondtransfer passage 192 feeds the second inlet port 195. The exhaust portof the'working part is indicated at 196 and the axis of symmetry of theworkingpart by the line 197. An auxiliary inlet port 198 is provided onthe axis of symmetry opposite to the exhaust port 196 and is fed fromthe transfer passage 191 by an auxiliary transfer passage 199.

It will be seen that the invention provides a number of differentarrangements of a stepped piston engine in which the transfer passagesare arranged symmetrically and enable a compact engine to be obtainedwith efficient scavenging and a symmetrical arrangement of the exhaustports and therefore of the exhaust pipes which is important for a motorcycle engine. It is possible also to make the transfer passages feedingeach. cylinder of substantially equal length which gives balanced flowand efficient scavenging and charging of the cylinders.

I claim:

1. A multicylinder, two-stroke engine comprising at least one set of twocylinders, each cylinder having a working part of smaller diameter, anda pumping part of larger diameter; a piston slidably reciprocable ineach cylinder and having a working part which slides in the working partof the cylinder and a pumping part which slides in the pumping part ofthe cylinder; transfer passage means between the pumping part of eachcylinder of the or each set and the working part of the other cylinderof the set to conduct charges from the pumping parts to the workingparts during operation of the engine; port means in each working partand comprising exhaust port means and first and second inlet port meansarranged symmetrically about the longitudinal axis of the cylinder withrespect to the exhaust port means and a deflecting surface formed by thecylinder wall opposite the exhaust port means; the transfer passagemeans for the or each set comprising four transfer passages which areseparate for at least the major portions of their lengths and consist oftwo first transfer passages extending respectively between the firstinlet port means of the first and second cylinders and the pumping partsof the second and first cylinders and two second transfer passagesextending respectively between the second inlet port means of the firstand second cylinders and the pumping parts of the second and firstcylinders, each cylinder being interposed between parts of the first andsecond transfer passages which feed the working part of-that cylinder,the first transfer passages of the set mutually crossing and the secondtransfer passages of the set mutually crossing, all the four transferpassages for a set of cylinders'being substantially similar so that theamounts of charge introduced through the first and second inlet portmeans of a working part in a cycle are substantially equal and enter theworking part at substantially equal speeds, and portions of the firstand second transfer passages immediately adjacent to the first andsecond inlet port means of each working part being disposed so that theflow paths of the charge entering said working part through said firstand second inlet port means coverage and meet adjacent said deflectingsurface and so that said surface directs the charge toward the cylinderhead of said working part.

2. An engine according to claim 1 having a first part with one of saidfirst and second parts. I I

4. An engine as claimed 1n claim 1 comprising a first part constitutedby a crankcase and cylinder block, first and second recesses formed insaid first part and respectively communicating with the inlet port'meansand the pumping parts of the cylinders and opening to the external wallsof thefirst part and crossover means releasably connected to the firstpart and interconnecting the first and second recesses to form therewiththe transfer passages.

5. An engine as claimed in claim 4 wherein said crossover meanscomprises a recessed second part and an apertured third part interposedbetween'the first and second parts, the apertures in the third partbeing disposed to place selected recesses of the first and second partsin communication such that the transfer passages are formed by therecesses and apertures.

6. An engine according to claim 1 wherein each working part has anauxiliary transfer port connected to one or both of the transfer passageor passages feeding the working part by an auxiliary transfer passage oflesser cross-sectional area than the or each transfer passage from whichit is fed and opening into the working part through said auxiliary inletport which is opposite to the exhaust port means.

7. 'An engine according to claim 1 wherein the first and second transferpassages of each working part are mutually connected by a passage oflesser cross-sectional area than the transfer passages to equalize thepressure in the transfer passages.

8. An engine according to claim 1 including a crankcase for the or eachset made in two halves having mating faces, at least parts of thetransfer passages being formed in said halves and being arranged so thatthe passage parts mate when the two halves are secured together.

9. An engine according to claim 1 wherein each set of cylinders comprisea crankcase having recesses formed therein, a cylinder block meansformed with recesses therein and a plate interposed between thecrankcase and the cylinder block means, the plate and the recessescooperating to form at least parts of said four separate transferpassages.

10. An engine according to claim 9 wherein the swept volumes of thepumping parts of the cylinders are provided in the crankcase and theworking parts each in a a separate cylinder block.

11. An engine according to claim 9 wherein where the passages cross theyare separated by the plate.

nun,

1. A multicylinder, two-stroke engine comprising at least one set of twocylinders, each cylinder having a working part of smaller diameter, anda pumping part of larger diameter; a piston slidably reciprocable ineach cylinder and having a working part which slides in the working partof the cylinder and a pumping part which slides in the pumping part ofthe cylinder; transfer passage means between the pumping part of eachcylinder of the or each set and the working part of the other cylinderof the set to conduct charges from the pumping parts to the workingparts during operation of the engine; port means in each working partand comprising exhaust port means and first and second inlet port meansarranged symmetrically about the longitudinal axis of the cylinder withrespect to the exhaust port means and a deflecting surface formed by thecylinder wall opposite the exhaust port means; the transfer passagemeans for the or each set comprising four transfer passages which areseparate for at least the major portions of their lengths and consist oftwo first transfer passages extending respectively between the firstinlet port means of the first and second cylinders and the pumping partsof the second and first cylinders and two second transfer passagesextending respectively between the second inlet port means of the firstand second cylinders and the pumping parts of the second and firstcylinders, each cylinder being interposed between parts of the first andsecond transfer passages which feed the working part of that cylinder,the first transfer passages of the set mutually crossing and the secondtransfer passages of the set mutually crossing, all the four transferpassages for a set of cylinders being substantially similar so that theamounts of charge introduced through the first and second inlet portmeans of a working part in a cycle are substantially equal and enter theworking part at substantially equal speeds, and portions of the firstand second transfer passages immediately adjacent to the first andsecond inlet port means of each working part being disposed so that theflow paths of the charge entering said working part through said firstand second inlet port means coverage and meet adjacent said deflectingsurface and so that said surface directs the charge toward the cylinderhead of said working part.
 2. An engine according to claim 1 having afirst part with recesses formed therein communicating with the inletport means, at least one second part having recesses formed therein andan apertured third part interposed between the first part and the secondpart, the apertures in such third part being disposed to providecommunication between selected recesses in said first and second partssuch that the transfer passages are formed by said recesses andapertures.
 3. An engine as claimed in claim 2 wherein each third part isalso provided with recesses which are aligned with recesses in one ofsaid first and second parts.
 4. An engine as claimed in claim 1comprising a first part constituted by a crankcase and cylinder block,first and second recesses formed in said first part and respectivelycommunicating with the inlet port means and the pumping parts of thecylinders and opening to the external walls of the first part andcrossover means releasably connected to the first part andinteRconnecting the first and second recesses to form therewith thetransfer passages.
 5. An engine as claimed in claim 4 wherein saidcrossover means comprises a recessed second part and an apertured thirdpart interposed between the first and second parts, the apertures in thethird part being disposed to place selected recesses of the first andsecond parts in communication such that the transfer passages are formedby the recesses and apertures.
 6. An engine according to claim 1 whereineach working part has an auxiliary transfer port connected to one orboth of the transfer passage or passages feeding the working part by anauxiliary transfer passage of lesser cross-sectional area than the oreach transfer passage from which it is fed and opening into the workingpart through said auxiliary inlet port which is opposite to the exhaustport means.
 7. An engine according to claim 1 wherein the first andsecond transfer passages of each working part are mutually connected bya passage of lesser cross-sectional area than the transfer passages toequalize the pressure in the transfer passages.
 8. An engine accordingto claim 1 including a crankcase for the or each set made in two halveshaving mating faces, at least parts of the transfer passages beingformed in said halves and being arranged so that the passage parts matewhen the two halves are secured together.
 9. An engine according toclaim 1 wherein each set of cylinders comprise a crankcase havingrecesses formed therein, a cylinder block means formed with recessestherein and a plate interposed between the crankcase and the cylinderblock means, the plate and the recesses cooperating to form at leastparts of said four separate transfer passages.
 10. An engine accordingto claim 9 wherein the swept volumes of the pumping parts of thecylinders are provided in the crankcase and the working parts each in aa separate cylinder block.
 11. An engine according to claim 9 whereinwhere the passages cross they are separated by the plate.